The subject matter of this application is directed to digital-to-analog converters (DACs), and more particularly to mitigating intersymbol interference (ISI) in DACs by accounting for parasitic capacitances in the DAC elements.
DACs receive a digital input signal and provide an analog output signal (e.g., current, voltage or electric charge) representing the digital signal. The DAC (e.g., a fully decoded DAC or a thermometric DAC) may include multiple unit elements providing a current or voltage to a DAC output. In one design, the unit elements may include active current sources which are controlled by the switches to provide a current at the output. In another design, the unit elements may include resistors connected to a reference voltage to generate the currents at the output. Depending on the value of the digital input signal, switches are operated to control the contribution of each unit element to provide an output current. The outputs of the unit elements are combined to provide an analog output signal representing the digital input signal.
As the switches are turned on or off, the parasitic capacitances associated with the elements may draw charge from the outputs (e.g., virtual ground nodes of the subsequent amplifying stage). Because in each clock cycle different unit elements may be activated and deactivated to provide the analog output signal corresponding to the digital input signal, the amount of charge drawn from the outputs may vary based on the change in the unit element states. The changes in the charge may draw a code dependent charge at the DAC output. The additional charge at the output may be the source for intersymbol interference (ISI), which is a form of distortion when error is dependent on the present code and previous codes.